The present invention relates in general to automatic control of an HVAC blower speed during shutoff of a start/stop engine in a motor vehicle.
Fuel economy of automobiles is an important attribute of vehicle performance which is determined by the technologies employed in the vehicle design, by driver behavior and actions, and by conditions under which the vehicle is used (e.g., speed, road design, weather, and traffic). Manufacturers continuously strive to deliver better fuel economy. One technology being increasingly used is known as automatic Start-Stop technology, wherein an internal combustion engine automatically shuts down when the vehicle comes to a stop or coasts and then restarts as needed to continue driving. The reduction in the amount of time the engine spends idling (e.g., while waiting at a traffic light) results in improved fuel economy and reduced emissions. According to some estimates, start-stop technology can provide a 5% to 10% improvement in fuel economy or more.
In addition to vehicle propulsion, the combustion engine drives other vehicle systems such as an air conditioning compressor. Occupant comfort must be maintained during the time that the engine is stopped. Since the air conditioning compressor typically runs on a front-end-accessory-drive (FEAD) belt driven by the engine, the conventional compressor does not run when the engine is stopped. Thus, when the air conditioning system is actively being used and the engine stops during an idle condition, the cooling action is interrupted and the passenger cabin may become warmer. If the cabin temperature increases by a certain amount, the engine is usually restarted so that cooling resumes, but some of the fuel economy improvement may be lost. One example of a strategy for controlling the engine off time is provided in commonly assigned, co-pending application U.S. Ser. No. 13/561,328, filed Jul. 30, 2012, entitled “Engine Start-Stop Control Strategy for Optimization of Cabin Comfort and Fuel Economy,” which is incorporated herein by reference in its entirety.
In attempting to lengthen the time span until it becomes necessary to resume operation of the air conditioning system, the use of cold storage systems has been considered. In one type of cold storage system, an evaporator may incorporate a phase change material that gives off heat (e.g., freezes) during normal operation before a stop event and then absorbs heat by changing back to a liquid phase during the stop event. However, cold storage devices are expensive, are difficult to package due to their larger size, and require additional controls. Moreover, since they consume additional energy during engine operation, the fuel economy improvement is lessened.
Another approach for providing air conditioning while the combustion engine is off involves the use of an electric compressor running off stored electrical energy from a battery. In a typical gasoline-powered vehicle, however, the expense of such an auxiliary air conditioning system is usually prohibitive. Even in a hybrid vehicle (i.e., having both a combustion engine and an electric propulsion system), the additional use of the electric compressor would result in the loss of fuel economy. Thus, it would be desirable to maintain passenger comfort with longer engine off times without relying on cold storage or backup cooling systems.